Aromatic amines are important intermediates which have to be produced inexpensively and in large amounts. Production plants for aromatic amines are for this reason generally built for very large capacities. The high productivity of these plants is ensured by very long reaction cycles and the problem-free operation between the starting-up and shutting-down operations for the hydrogenation in order to regenerate the hydrogenation catalysts used.
Aniline is an important intermediate, e.g. for the preparation of methylenedi(phenyl isocyanate) (MDI) and is generally prepared on an industrial scale by catalytic hydrogenation of nitrobenzene by means of hydrogen. Particular preference is given to reaction conditions as described in GB 1 452 466 A1, EP 0 011 090 A1 or EP 0 944 578 A2 (isothermal mode of operation) and in EP 0 696 574 B1, EP 0 696 573 B1, EP 1 882 681 A1 (adiabatic mode of operation). Apart from the abovementioned processes having stationary catalyst beds, processes having fluidized catalyst beds have also been described, e.g. in DE 1114820 B, DE 1133394 B or WO 2008034770 A1.
In all the adiabatic and isothermal processes described, the starting material nitrobenzene is reacted with an excess of hydrogen.
The preparation of the aromatic amines is carried out in reaction cycles because the catalytic activity of the hydrogenation catalysts decreases steadily.
The activity of used catalysts for the hydrogenation of aromatic nitro compounds therefore has to be restored at periodic intervals. For this purpose, a regeneration in which carbon-containing deposits are removed from the catalyst by burning-off in a stream of air is carried out. In other embodiments of the process, the burning-off step is followed by a washing step as described in, for example, U.S. Pat. No. 3,684,740. The next reaction cycle can then be started by starting up the hydrogenation plant again. Processes with multiple washing are also described in WO 2012013677 A1.
All the literature references cited do not describe the starting-up process and its difficulties.
In a process for hydrogenating nitro compounds to form the corresponding amines as described in EP 0 944 578 A2 (page 2, lines 1-20), the start-up procedure is discussed. There, it is stated that it is advantageous in terms of the space-time yield to increase the space velocity of aromatic nitro compounds introduced over the catalyst continuously or stepwise to the maximum space velocity over a period of from 10 to 1000 hours.
The quality of a process for hydrogenating aromatic nitro compounds is, firstly, defined by the amount of undesirable by-products of the reaction in the product. Secondly, the quality of a hydrogenation process is defined by the total process of hydrogenation cycle, shutting-down of the hydrogenation, regeneration of the hydrogenation catalyst and start-up of the hydrogenation process being able to be carried out without technical production stoppage.
Although the processes of the prior art which have been described allow aromatic amines having a low content of by-products, which thus, for example, contain only from 50 ppm to 300 ppm of phenols, to be prepared, these by-products always include products which are formed by liberation of ammonia (deamination reaction). This ammonia can react with other compounds (for example CO2) in the reaction system or the offgas lines and form deposits. Separating off ammonia-containing offgas streams and disposing them separately (for example separate introduction into a thermal waste air purification) requires an increased outlay in terms of apparatus and is not always completely possible. The possible problems during start-up of a process for preparing aromatic amines are passed over.